There is an increasing requirement in the field of optical communications and optical logic circuitry to develop simple optical components to achieve functions such as switching, logic operations, amplification and the like.
In accordance with one aspect of the present invention, an optical device comprises an optical waveguide formed from an assembly of optically conductive media having different refractive indices, at least one of which is non-linear, as hereinafter defined whereby an optical signal with a single mode is guided or not guided along the waveguide in accordance with the intensity of the signal; and coupling means for coupling at least two optical signals into an input end of the waveguide.
In accordance with a second aspect of the present invention, a method of operating such an optical device comprises injecting a first optical signal into the waveguide, the intensity of the first signal being such that the signal is substantially cut-off; and injecting a second optical signal into the waveguide with an intensity such that the resultant of the first and second signals has an intensity sufficient to be guided by the waveguide.
This invention use of the properties of inhomogeneous media having a non-linear refractive index such that the waveguide switches from a guiding to a non-guiding condition depending upon the intensity of the total incident light. This self-guiding property can be used in a number of different applications.
Preferably, the waveguide comprises at least three nested media, the two inner media having refractive indices in the guiding condition respectively greater and less than the refractive index of the outer medium, the refractive index of at least one of the inner media being non-linear.
Typically, the waveguide comprises a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding.
For example, a typical optical fibre has a core with a refractive index higher than that of the outer cladding, and an inner cladding with a refractive index lower than the outer cladding.
An example of a suitable optical fibre is a fibre having a W profile which has a fundamental mode cut-off which can be moved to the desired wavelength by suitable design. Silica has a self-focussing Kerr coefficient which would require powers of about 100 kW. Other materials which could be used include doped silica and silica W capilliaries containing highly non-linear organic materials.
By "non-linear" we mean that the refractive index of the medium (n.sub.1) varies with intensity. Typically, this will be according to the formula: EQU n.sub.1 =n.sub.0 +n.sub.2 .vertline.E.vertline..sup.2
where n.sub.0 is the linear refractive index of the medium, n.sub.2 is the Kerr coefficient, and .vertline.E.vertline..sup.2 is the intensity of the incident light.
Typically, .vertline.E.vertline..sup.2 n.sub.2 is very much less n.sub.0 and thus the non-linear effect is only apparent at high intensities. The invention is concerned with waveguides where the intensity of the incident light and the Kerr coefficient are such that n.sub.0 is preferably not more than two to three orders of magnitude greater than n.sub.2 .vertline.E.vertline..sup.2.
It should be understood that the refractive index n.sub.2 may increase or decrease with intensity.
In a homogeneous medium the non-linear refractive index of the waveguide will cause a transverse optical field to disperse at low intensities but to collapse to a self-focussing singularity at high intensities. In other media, a self-defocussing non-linearity will occur where the refractive index decreases with intensity. The type of non-linearity depends upon the sign of the Kerr coefficient. In an inhomogeneous medium it is possible for guiding to occur before catastrophic self focussing occurs.
The device is operated in the following way for a waveguide with a self-focussing non-linearity; if a waveguide with a self-defocussing non-linearity is used then the words in brackets apply. At a frequency near to the fundamental mode cut-off the optical field is not well guided. At low (high) powers most of the power launched diffracts out into the cladding. As the intensity is increased the refractive index derived from the non-linearity causes the cut-off to move to lower (higher) frequencies. This means that the optical field at the operating frequency is more (less) guided than before and hence that less (more) power diffracts into the cladding. If the power confined in some central region is defined as the output power then we have a device whose output power is a non-linear function of the input power.
In other words the guiding/non-guiding property depends on the resultant refractive index experienced by the optical field.
Preferably all the media are non-linear.
Preferably, the device further comprises separation means associated with an output end of the waveguide whereby portions of optical signals leaving a core region of the output end are separated from other portions of the signals. By providing separation means, the variation of output power with input power just described can be utilised. Typically, the separation means may comprise a monomode optical fibre which, in the case where the optical waveguide comprises an optical fibre, is spliced to the optical waveguide. In this way, only that portion of the optical signal passing through the core of the optical waveguide is coupled into the monomode optical fibre as the output signal.
Conveniently, the device further comprises bias signal generating means, such as a laser, for injecting optical bias signal into the input end of the waveguide, the intensity of the bias signal being such that the waveguide operates in its non-linear region and the bias signal is cut-off. This enables the device to be used in a variety of applications. For example as a switch, the device, when biassed into its cut-off region, can be caused to switch on or off by applying a suitable switching signal in addition to the bias signal causing optical radiation to be guided or not respectively. The bias and the signal must be phase aligned.
The device can also be used as the basis of a logic element such as an AND gate in which a number of inputs (two or more) are applied to an input end of the optical waveguide and optical radiation is only received at an output end of the waveguide if the total incident intensity is sufficient to cause self-guiding to occur. It will be appreciated that in these applications the optical waveguide must have a suitable length. A device with a self-defocussing non-linearity could be used as the basis of a NAND gate.
Conveniently, the device further comprises coupling means such as a Y coupler, one input arm of the coupler being coupled with the bias signal generating means, the other input arm of the coupler being coupled with an input signal, and the output arm of the coupler being coupled with the waveguide.